With the recent proliferation of hybrid automobiles that can run on both energies of gasoline and electricity, uninterrupted power supplies, mobile communication devices, portable electronic devices, and the like, the requirement for improving the performance of chargeable and dischargeable power storage devices has been greatly increased. Specifically, there are requirements for a high output power, a high capacity, and improvement in charge-discharge cycle characteristics.
For example, Patent Documents 1 and 2 propose, with the purpose of achieving a high output power and a high capacity, a power storage device in which a positive electrode including an organic compound having a conjugated π-electron cloud as an electrode active material and a negative electrode as conventionally used for lithium batteries are used in combination.
In the power storage device in which an organic compound with low molecular weight is used as an electrode active material, however, the electric capacity may be reduced with repeated charging and discharging. This is presumably because the molecules of the organic compound serving as the active material are partially dissolved into an electrolyte from the electrode as charging and discharging are repeated, and separated from the interior of the electrode, and the separated active material cannot contribute to the charge-discharge reaction.
Patent Document 3 proposes using in a battery in which an electrically conductive organic complex formed of a positively or negatively charged organic compound and an ion contained in an electrolyte having a polarity opposite to the polarity of the organic compound is used as an electrode active material, an electron donative or electron receptive organic compound as used for electrocrystallization (electrochemical crystal growth method), as the organic compound constituting the conductive organic complex.
The electrocrystallization (electrochemical crystal growth method) is a method in which two electrodes are introduced in a solvent prepared by dissolving the organic compound and a supporting salt for the electrolyte, and voltage is applied across the two electrodes, thereby to cause an oxidation or reduction reaction on one of the electrodes, so that oxidant crystals or reductant crystals are formed. Patent Document 3 further discloses a perylene perchlorate complex, a tetrathianaphthalene perchlorate complex, and the like as the examples of the conductive organic complex, and using these as an electrode active material.
The use of a perylene perchlorate complex as an electrode active material is described with reference to FIG. 14. FIG. 14 is a schematic diagram of a battery in which a perylene perchlorate complex is used as an electrode active material. The battery has an electrolyte 51 prepared by dissolving lithium perchlorate serving as a supporting salt in tetrahydrofuran serving as a solvent, a positive electrode 52 being a platinum electrode, a negative electrode 53 made of metallic lithium. The electrolyte 51 further includes perylene serving as a positive electrode active material. In charging, the perylene dissolved in the electrolyte is oxidized on the positive electrode 52, whereby a perylene perchlorate complex (solid) is produced. In discharging, the perylene perchlorate complex (solid) precipitated on the positive electrode 52 is reduced to be perylene and dissolved in the electrolyte. Here, the reaction that occurs on the negative electrode 53 is a dissolution-deposition reaction of metallic lithium.
As such, in the foregoing battery, the active material is dissolved in the electrolyte in discharging, and therefore the following three serious problems arise when the active material is actually used in a secondary battery.
First, the battery reaction occurs on the surfaces of the electrodes. Since the positive electrode active material in the foregoing battery is dissolved and dispersed in the electrolyte, the positive electrode active material present away from the surfaces of the electrodes is not utilized for the battery reaction. This means that the ratio of the perylene that can contribute to the charge-discharge reaction to the whole perylene dissolved in the electrolyte is small, and as a result, the capacity density as a power storage device is significantly reduced.
Secondary, as in the case of the foregoing battery, when the battery reaction involves dissolution and deposition of an active material, a secondary battery having excellent charge-discharge cycle characteristics is extremely difficult to obtain. A needle-like deposit called dendrite is formed and grown during the deposition, and as charging and discharging are repeated, the grown needle-like deposit may be broken and separated from the electrode plate, and thus can no more be used for charging and discharging. Alternatively, the needle-like deposit may be grown on one of the electrodes until it reaches the other one of the electrodes, causing the positive electrode and the negative electrode to be short-circuited. As a result, the battery may fail to operate normally.
These phenomena are well known in non-aqueous batteries including a metallic lithium electrode, the battery reaction mechanism of which involves dissolution and deposition.
Thirdly, the organic compound capable of being oxidized and reduced, such as perylene, is dissolved in the electrolyte and brought in contact with both the positive electrode and the negative electrode, causing the positive electrode and the negative electrode to be short-circuited. Once the positive electrode and the negative electrode are short-circuited, self-discharge proceeds during the storage of the battery, and thus the amount of charged electricity of the battery is reduced.
As described above, although the use of the conductive organic complex formed of the cation or anion of the organic compound and the anion or cation of the supporting salt in the electrolyte as an electrode active material has been disclosed, it has been difficult to simultaneously achieve both a high capacity and improvement in charge-discharge cycle characteristics in secondary batteries.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-111374    Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-342605    Patent Document 3: Japanese Laid-Open Patent Publication No. Sho 60-14762